I did a hot water test of my cooler with aprox 18L. The water had an initial temperature of 71.3'C which sunk to 65.5'C in 70 minutes. I did not preheat the cooler but let it approach equilibrium for about 5 minutes before taking the temperature. The temperature sunk from 65.5'C to 61.6'C during the next 70 minutes. I placed a blanket on top of the lid for better insulation. Using the last measurement that equals a loss of about 3.3'C or 6'F per hour.

Almost all the starch conversion happens in the first 20-30 minutes of the mash, the rest of the time you are just making "tea" -- getting color and flavor. Once that first 30 minutes passes, holding that perfect temperature is not as critical.

For my cooler I built a heat exchanger that uses steam from a pressure cooker and I use a March pump to recirculate. This way I can heat the mash without boiling water or direct heat if I need to.

I think I'll do it the easiest way possible first (never done all-grain before). Making a RIMS sounds like a lot of fun (I love electronics, computers and mechanics) but will have to wait til after some decent beer has been made, drunken and praised

What kind of insulation is typical for drink coolers (mine is a Coleman)? Would I benefit from replacing the possibly old insulation with polyurethane spray foam (R-value of 5.5-6.5) or should I just leave it as-is? Coolers of this size is incredibly expensive and hard to get where I live so I would really like to just stick with what I gathered for free.

The cooler insulation should be fine. Do you add boiling water to it first to pre-heat it? That can help. I also made a rigid foam lid that I can push down so it's right over the grain bed reducing the airspace to lose heat to.

As said the calculation of -3.3'C/h where from the second measurement where the cooler already had been standing with hot water in it for over an hour (aka preheat). Still, insulating the lid would be advantageous as it's the least insulated wall of the container.

Honestly, I stopped checking my final temp when I do cooler mashes. If you lose a few degrees over an hour it's not a big deal. If you want to hold in more heat, try taping the edges of the lid and wrap the whole thing in a blanket. Most of your heat loss is probably coming from the lid though. Mine doesn't fit air tight and I can see steam coming out around it. Also, I've found pre-heating with boiling water to be an extra step that I just don't like to do. Instead, I made adjustments to my strike equation to compensate for the initial heat loss to the tun.

__________________Slots Down BreweryStick with the plan....not the sparge.

my first 5 gallon batch i did I only used a plastic bucket wrapped in a blanket and It worked phenomenally. My next batch i had already purchased a cooler and had more heat loss with that however it still worked well. Most likely next time to be on the safe side I will also wrap my cooler with the blanket.

Alright, it seems I should not worry. Thanks for the input, much appreciated I'm probably going to install a digital temp. probe and can repost after the first batch. As far as I remember from biological chemistry enzyme activity will decrease below/above a prefered temperature. Beer chemistry is somewhat complex but still, it's been made for ages without much of modern scientific research..